If a rental property is vacant part of the year, how should I answer"was the rental rented every day of the year"?

If it was available for rent but not rented then answer Yes to the question.

The property is considered rented the whole year if:

1. You were at least attempting to rent it during it's period of vacancy and;

2. You did not live in the property for one single day as your primary residence, 2nd home, vacation home or any other type of "personal pleasure" use.

Note that if you didn't advertise for say, a week or two after the last renter moved out because you needed that time to prepare it for the next renting, then that time still counts in your favor. This would also hold true if it was for say, a month or two while you performed major renovations or had major damage repaired in order to make the property move in ready for the next renter.